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computer compatible

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gmcjimmy90

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hello everybody new member. i have a 1990 gmc jimmy(fullsize) it has 155000 miles on it i am planning on going through the motor this winter and was wondering if anyone can tell me how big of cam can i go with and how much compression with out giving the computer to much of a fit looking to get 350-400 horses out of it. this is my daily driver motor work not a problem i have drag racing for 10yrs now and building my own engines for 8yrs. but the computer has me wondering. thanks in advance.
 
not knowing what size engine you have or if it even has factory roller cam i can only guesstimate that a dual pattern bumpstick with 206/212 @ .050, 258/264 advertised, .480/.488" lift on a 112 LSA would be the limit with TBI and a stock computer.
The valve springs wont accept the lift without changing,
i think factory Vortec heads (do you have vortec's.?) are good for .480" lift max.
If you want to use a standalone EFI controller (http://www.megasquirt.info/ms2/megasquirtleft.htm) your choices are much less limited.
I would not run over 9.5:1 on a street motor.
Remember that duration at .050" must be compatible with the static compression ratio (It's a cylinder pressure thing)
 
EFI cam

The trick with that is to put in a cam that will still generate enough of a vaccuum signal that will not cause a MAP or a MAF code, so you would need to increase the lobe seperation angle (LSA) as the duration is increased, say, from 110 to like 115. If you are partial to a specific brand of cam, I would call their tech line (Usually an 800 #) and they could recommend a cam that will give you some added HP and still not need a computer reflash. The added compression will help the vaccuum signal stay steady with the increased cam specs. Even so, you may want to add a chip or a reflash to your future plans, since you're probably going to headers, a manifold, etc., etc., and that will help the PCM get out most out of your upgrades. :waytogo:
 
I am also in agreement with you on the theory of a tight lobe center (108) being more compatible with computer controls than a similar cam with a wider LSA (112)
 
Lsa

They keep the LSA wide on EFI cams to keep the overlap low so the engine will generate a decent vaccuum signal. We seem to be of the same mind on alot of things there Mad dog, as I have read your posts in other threads asking advice. 2 great minds that think alike? :thinking: :whistle:
 
it has a 350 tbi non roller and not vortec(i wish it was). it is gutless pulls great just has no go. came from factory with 210hp.i think that the 488 cam is a little bigger than what i was thinking.i was looking at more along the lines of 460lift. what kind of upgrades to the computer should br done. thanks for the info.
 
The only thing I knows to do with that is buy a DFI system. Or my other sugestion is something I have done, Rip out computer and tbi and install carb. :laugh: DFI is fully adjustable and in the future upgrades you would already have it. Some guys are running this on stock engines.
 
Carb vs. EFI

That may not be an option for him if he has to deal with the smog police. I'm sure if you have a stock exhaust that it's robbing you of alot of power by being so restrictive, or your cats are plugged.
 
i have thought of putting carb on but i do like the fuel injection( first one i have had). always have owned older trucks and muscle cars. smog not a problem true duals, headers, 2 1/2" exhaust flowmasters and the cat well lets just say it ran away.
 
Efi

If you're rig is a 1990, it's an OBD I system, which is basically what every aftermarket EFI setup is. (OBD II is 96 and later) With the exhaust system you have now, I'm pretty confident you can call a cam company, and they can recommend a cam for you that'll work with what you have now. Why go out and buy an aftermarket version of what you already have now anyway? You can always get the computer reflashed/rechipped alot cheaper, usually to match whatever cam you want to run. I'm partial to Comp Cams, but you'll here a lot of guys who like other brands too. Call a few and see what they have to say, and go from there. The more info you can give them, the better. Stuff like what heads you have, gear ratios, how much the vehicle weighs, what you intend to use it for, what you ACTUALLY use it for in real life, you get the idea. Good luck.
 
You can have your cake and eat it to with a SBC.
That particular engine has some serious air flow problems
with the TBI unit, intake manifold and the heads themselves.
Chance are the air/fuel ratio is already calibrated on the lean side so any mods that you make will further attempt to lean the mix.
You will have to upgrade the PROM (programmable read only memory) chip at the very least in a attempt to take advantage of the mods you listed.
 
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